1. Field of the Invention
This invention generally relates to an electronic device, and in particular, relates to an electronic device having two transmission lines.
2. Description of the Related Art
A micro strip line, a coplanar line and so on are used as a transmission line for high frequency wave such as a micro wave, a milliwave and so on, the micro strip line having a dielectric layer provided between a signal line and a ground, the coplanar line having a signal line and having grounds provided on both sides of the signal line.
Japanese Patent Application Publication No. 10-256801 discloses a case where two micro strip lines are coupled. A description will be given of a conventional embodiment, with reference to FIG. 1A and FIG. 1B. FIG. 1A illustrates a top view of the conventional embodiment. FIG. 1B illustrates a cross sectional view taken along a line B-B of FIG. 1A. As shown in FIG. 1A and FIG. 1B, a first micro strip line 10 and a second micro strip line 20 are coupled to each other. The first micro strip line 10 has a structure in which a first ground layer 16 is provided on a back face of a first dielectric layer 14 and a first signal line 12 is provided on a front face of the first dielectric layer 14. The second micro strip line 20 has a structure in which a second ground layer 26 is provided on a back face of a second dielectric layer 24 and a second signal line 22 is provided on a front face of the second dielectric layer 24.
The first ground layer 16 and the second ground layer 26 are arranged on a metal substrate 50 composed of a conductive material such as copper and are fixed to the metal substrate 50 so as to be electrically coupled to each other. The first signal line 12 and the second signal line 22 are coupled to each other through a bonding wire 30a. Thus, the first ground layer 16 and the second ground layer 26 are electrically coupled to each other without a generation of parasitic inductance. Therefore, an electrical potential of the first ground layer 16 is equal to that of the second ground layer 26 from a view point of DC and high frequency wave.
However, in a case where the first ground layer 16 and the second ground layer 26 cannot be arranged on the metal substrate 50 as a case of the conventional embodiment, the parasitic inductance is added when the first ground layer 16 and the second ground layer 26 are coupled to each other through a bonding wire and so on. The parasitic inductance is not negligible even if the length of the bonding wire is reduced, when the frequency of a high frequency wave is higher. That is, the electrical potential of the first ground layer 16 is different from that of the second ground layer 26 and it is difficult to match impedances of two micro strip lines. Therefore, a loss is increased. And a mismatch is generated between the impedances, because the first signal line 12 and the second signal line 22 are coupled to each other through the bonding wire 30a. 